1. Field of the Invention
The present invention relates to a process and system for producing energy through a thermodynamic cycle. Specifically, the invention relates to a process and a system for implementing a thermodynamic cycle that utilizes a working fluid having at least two components, wherein at least a portion of an exhaust stream from at least one turbine in a turbine train is diverted to heat a feed stream to a heater that provides additional heat to the feed stream prior to it entering the turbine train. This portion of the exhaust stream is thereby partially condensed, and the liquid is removed to form a vapor stream that is returned as feed to a subsequent or downstream turbine in the turbine train for further expansion.
2. Description of Related Art
In Rankine cycles for power generation applications, the conventional working fluid is superheated steam created by the evaporation of water. The heat requirement for evaporation of water is usually large, and the liquid-to-vapor phase change requires a large amount of thermal energy. A proportion of the latent heat is recovered by extracting steam from the turbine stages following expansion and using it to preheat the boiler feed water. This is sometimes referred to as regenerative heating. The final discharge steam usually passes to a condenser and remaining latent heat is removed by cooling water and is not converted into usable energy. The extent of this unrecovered latent heat is one of the factors limiting steam cycle efficiency.
Higher steam pressures generally result in higher cycle efficiency, but since these higher pressures also increase the boiling point of the water, the temperature of the medium providing the heat also has to be at a higher temperature. This means that regenerative heating using extraction steam is limited mainly to heating water with very little potential for generating steam.
Using a multi-component working fluid in a Rankine cycle that consists of two or more components having suitable thermodynamic and solubility characteristics, such as an ammonia/water vapor mixture, offers advantages over water/steam alone. The heat required for evaporation of ammonia/water is lower than that of water, so less energy is required to evaporate the liquid working fluid. Also, the boiling point of ammonia/water is lower than that of water, thus allowing regenerative heating to supply more of the evaporative duty to produce the final working fluid. As more of the latent heat is used for heating the ammonia/water working fluid, less energy is rejected in the condenser and the cycle efficiency is increased. The mixture used usually is ammonia rich, but the exact concentration used will depend upon the operating characteristics of the cycle employed.
Various attempts have been made to improve efficiencies of thermodynamic cycles, such as Rankine cycles using ammonia/water vapor mixtures as working fluids. For example, U.S. Pat. No. 4,899,545 (the ""545 patent), incorporated herein by reference in its entirety, discloses a method and apparatus for implementing a thermodynamic cycle that includes the use of a composite stream having a higher content of a high-boiling component than a working stream to provide heat needed to partially evaporate the working stream. The working stream, after being partially evaporated, is completely evaporated with heat provided by returning gaseous working streams and heat from an auxiliary steam cycle. The working stream is then superheated and expanded in a turbine, with the expanded stream separated into a spent stream and a withdrawal stream. The withdrawal stream is combined with a lean stream to produce the composite stream, which partially evaporates the working stream and preheats the working stream and the lean stream. A first portion of the composite stream is fed into a distillation tower, from which a liquid stream flows and forms the lean stream. A second portion of the composite stream is combined with a vapor stream from the distillation tower to form a pre-condensed working stream, which is condensed to form a liquid working stream, which is preheated and partially evaporated to complete the cycle.
Thus, as disclosed in the ""545 patent, in an effort to achieve the alleged efficiency increase, a withdrawal stream is separated from the expanded stream, and an elaborate process, including combination of the withdrawal stream with a lean stream and use of a distillation tower, is employed to fully condense the withdrawal stream before sending it back as part of the working fluid.
U.S. Pat. No. 5,095,708 (the ""708 patent), incorporated by reference in its entirety, discloses a method and apparatus for converting thermal energy into electric power by expanding a high pressure gaseous working stream and producing a spent stream. The spent stream is condensed to form a condensed stream, which is then separated into a rich stream having a higher percentage of a low-boiling component and a lean stream having a lower percentage of the low-boiling component. The rich and lean streams each pass through a boiler, generating evaporated rich and lean streams, which are then combined to form the high pressure gaseous working stream. The ""708 patent alleges that the generation of two multi-component working streams allows for a better match of the required and available heat in the process, thus increasing thermal efficiency.
The foregoing technologies are complex and involve extensive modifications to be incorporated into standard boiler designs. Moreover, the efficiency gains offered by these technologies are considered insufficient to encourage general commercial acceptance. Therefore, there is still a need for a process and system for producing usable energy using a thermodynamic cycle in a more efficient and cost-effective manner. Furthermore, there is a need for a process and system for producing usable energy that can easily be adapted to use currently available systems, equipment and apparatus of existing thermodynamic cycles.
Accordingly, the present invention provides a process and system for producing usable energy through a thermodynamic cycle. The process and system produce usable energy, such as mechanical and electrical forms of energy, through novel implementation of a thermodynamic cycle that utilizes a working fluid having at least two components.
A specific feature of the present invention is a reduction or removal of condensate or moisture from an exhaust stream, or portion thereof, from a turbine or expansion stage within a turbine train. This allows the resulting vapor stream to be further expanded and provides additional heat to the feed stream or working fluid, thereby improving overall cycle efficiency. Thus, one feature of the present invention is that it may improve thermodynamic efficiencies using currently existing systems and equipment. Also, the present invention may be incorporated into new designs.
These benefits are provided by the present invention, which, in one embodiment, comprises a process for producing energy through a thermodynamic cycle comprising transforming a first working fluid having at least two components into usable energy and a first exhaust stream; diverting at least a portion of the first exhaust stream to form a diverted first exhaust stream; transferring heat from the diverted first exhaust stream to the first working fluid, thereby partially condensing the diverted first exhaust stream to form a partially condensed diverted first exhaust stream; separating the partially condensed diverted first exhaust stream into a vapor stream and a liquid stream; and transforming the vapor stream into usable energy. The present invention also comprises a system for producing energy through novel implementation of a thermodynamic cycle.
Other benefits and features of the invention will appear from the following description from which the preferred embodiments are set forth in detail in conjunction with the accompanying drawings.